PROJECT SUMMARY Age is the strongest risk factor for Alzheimer?s disease, and therefore, targeting factors that contribute to the onset of this disorder may be key for early prevention (1-3). Here, we propose that increases in neuroinflammatory CCL5/CCR5 chemokine signaling in middle-age(4-8) trigger the down-regulation of MAPK/CREB signaling(9-15), and that this contributes to the onset of memory deficits with aging and Alzheimer?s disease, including source and relational memory problems(16-20). Our memories depend not only on the ability to recall individual elements/items, but also on processes that link these elements/items (source/relational memory). Importantly, source/relational memory is more sensitive to age-related decline than item memory(16-20). Recently(21), our laboratory found that although in middle-aged(14-16 month old) mice contextual memory is still preserved, these mice show robust impairments in linking the memories for two contexts, a possible early sign of age-related memory deficits, including in Alzheimer?s disease. Two memories are linked when the recall of one triggers the recall of the other(21, 22). Accordingly, our studies showed that the CA1 neuronal ensembles activated by two linked contexts display significant overlap in young, but not in middle-aged mice(21). Thus, the deficit in ensemble overlap we found in middle-aged mice could underlie their memory linking deficits(21). Additionally, recent studies, including those carried out in the previous period of this project(23-34), are consistent with the hypothesis that learning triggers CREB activation and a subsequent temporary increase in neuronal excitability(35-37), that for a time biases the allocation of a subsequent memory to the neuronal ensemble encoding the first memory. We and others(21, 22) showed that this overlap between neuronal ensembles links these memories across time, such that the recall of one memory leads to the recall of the other. Our preliminary studies showed that a CCR5 null-mutation increases CREB signaling and rescues deficits in memory linking in middle-aged mice. Here, we propose to test whether increases in CCL5/CCR5-activation, and subsequent decreases in CREB-signaling, contribute to age-related decreases in CA1 excitability(Aim 1), whether this alters memory allocation in CA1(Aim 2), leading to the lack of overlap between CA1 neuronal ensembles encoding distinct contexts(Aim 2), and therefore, age-related impairments in memory linking(Aim 3). Importantly, we will test whether an FDA approved CCR5 inhibitor could be used to treat age-related memory and linking impairments. We will focus on CA1 because we showed that chemogenetically rescuing excitability in a subset of CA1 pyramidal neurons was sufficient to rescue contextual memory linking in middle-aged mice (21). Age-related increases in CCL5/CCR5 neuroinflammatory mechanisms(4-8) and their adverse effects on brain CREB signaling(38), could contribute to the onset of cognitive impairments in aging as well as in Alzheimer?s disease, since higher neuroinflammation(39) and beta- amyloid-dependent lower CREB levels, have been implicated in this disorder(40, 41).